Lens edging machines are well-known in the art, and different types are described in U.S. Pat. Nos. 4,870,784 and 5,148,637. A lens is formed from a blank having a certain curvature in accordance with the desired optical focusing power. The curvature of an ophthalmic lens provides a corrective focusing power. The lens curvature for dark glasses or sun glasses typically provides no corrective focusing power. Having formed the lens blank with the desired curvature, the lens is cut from the blank in a shape that fits into the frame of the glasses. This is accomplished by edging the blank, i.e., grinding the edges of the lens with an edging tool, such as a grinding wheel, until the desired lens shape is reached. If the lens is ophthalmic, the edging process may be performed by a lens edging machine of the type described in U.S. Pat. No. 4,870,784, which uses a groove in the grinding wheel to bevel the lens edge. Another type of lens edging machine typically employed to make non-ophthalmic lenses uses an apex in its grinding wheel to bevel the lens edge. The bevel on the lens edge enables the lens to fit tightly into the frames of the eye glasses or sun glasses. Both types of the lens edging machines rotate the lens blank with respect to the grinding wheel. As the lens blank is rotated, the machine simultaneously changes the displacement between the center of the lens blank and the wheel to achieve the desired shape for the lens.
After completion of the edging process, the lens is subject to a final inspection step. During inspection, the lens is compared to the design parameters used to make the lens. Such parameters include the length of the major and minor axes, the length of the periphery of the lens, the radius of curvature of the lens at points along the periphery, the bevel angle of the peripheral edge of the lens, etc. The inspection process is normally a manual operation wherein trained inspectors compare a sample of a manufactured lot of lenses to design standards.
More specifically, the size is measured using an eye plate and eye wire. The measuring device is initially zeroed using a hardened steel eye plate, one eye plate for every style for every inspection station. Then the eye wire is fitted over the lens to be measured, introducing the possibility of lens abuse in the process, and a reading is taken. Repeated measurement of the same lens by different inspectors may result in measurements that vary by 50 percent or more of the allowable size range. Next, bevel angle is checked, and to do this requires the use of a five-times comparator. The comparator is a magnifier through which a magnified image of the lens edge is viewed and the bevel angle measured using what is little more than an enhanced protractor. The inspection process is fairly subjective, especially so in the case of rounded apex lenses. Apex location is also checked on the five-times comparator, lining up the image with a reference line, zeroing the readout and winding across the edge of the bevel. Accuracy of this method of measurement is barely adequate, given the size of the specification; to perform an effective analysis, a seven-times loop with graticule really must be used. This provides a precision of .+-.25% of the allowable range.
Lastly, shape is validated by placing the lens on a 5x shadowgraph machine, which projects a magnified image of the lens onto the blueprim. First, the relevant blueprim is retrieved from its file and placed on the lab table. The trained operators, once again, use their subjective abilities to decide whether or not the lens corresponds accurately enough to the true line of the blueprint. If it does not, then the elevation from true shape is a pair of calipers to measure the distance between the edge of the shadow and the true line. Since the true line is at least 10/1,000th of an inch wide inspectors often fail to measure accurately the shape curve.
As such, the current manual process of inspection is time consuming, unreliable and inaccurate and is often ignored. This has an overall negative impact on the manufacturing process because errors are not timely detected and corrective action is often delayed. The latter results in substantial losses when lens fail to fit their frames and have to be scrapped.